The present invention relates to an image forming apparatus employing a toner which is prepared by coating the surfaces of resin mother particles with external additives.
In a full-color electrophotography, an intermediate transfer member is employed for the purpose of preventing wear of a photoreceptor and of facilitating superposition of multi-color images. In addition, used toners are small particle toners of 20 xcexcm or less or 10 xcexcm or less in diameter for giving improved resolution. The small particle toner has however poor transfer property compared to a large particle toner. If the transfer electric field is enhanced to improve the transfer, dielectric breakdown can unfortunately occur and be followed by image defects. The transfer property depends on the adhesive forces (van der Waals forces, image-forces) between the toner and the photoreceptor and between the toner and the intermediate transfer member.
In order to improve the transfer property, it has been therefore proposed that the adhesive force between the intermediate transfer member and the toner is set to be larger than the adhesive force between the photoreceptor and the toner and, in addition, the adhesive force between the toner and a receiving sheet is set to be larger than the adhesive force between the intermediate transfer member and the toner (Japanese Unexamined Patent Publication No. H05-503377).
By the way, a single-component non-magnetic toner is generally formed by coating the surfaces of resin mother particles with external additives to reduce the adhesive force of the toner and improve the charging property. In a developing device using such a toner, the toner is supplied to a development roller by a supply roller while being agitated and is regulated into a toner layer having a certain thickness on the development roller and triboelectrically charged by a regulating blade. In this manner, the toner is carried to a nip portion relative to the photoreceptor. In normal operation, toner particles which move from the development roller to the photoreceptor for the consumption may account for a several percent of the toner at the most. The rest is returned to the developing device again for the agitation and the triboelectric charging. The circulation of the toner is repeated. In this process, toner particles which are well coated with external additives and have good charging property are consumed prior to other particles (this phenomenon will be referred to as xe2x80x9cselective developmentxe2x80x9d).
As a result of such selective development, circulated toner particles in the developing device are deteriorated so that a part of the external additives may be embedded into the resin mother particle and a part of the external additives may be released from the surface of the resin mother particle to as to expose the resin surface of the mother particle. Even in a new toner, not all mother particles have surfaces sufficiently coated with external additives. This means that the toner includes, by a several percent, liberated mother particles insufficiently coated with external additives. As a result, the adhesive force of toner particles sufficiently coated with external additives significantly depends on the effects of the external additives while the adhesive force of toner particles in which the amount of external additives on toner surfaces is reduced according to the repetition of circulation and the adhesive force of liberated mother particles significantly depends on the effect of the resin mother particles.
The invention proposed in the aforementioned Japanese Unexamined Patent Publication No. H05-503377, however, has been made without taking such reduction in amount of external additives according to the repetition of circulation and such liberated mother particles into consideration. This is because the transfer efficiency could not be sufficiently improved, leading to reduction in transfer efficiency. In case of an apparatus with a cleaner, there may be no problem caused due to the reduction in transfer efficiency because the residual toner particles are removed. In case of an apparatus without a cleaner, i.e. conducting a cleaner-less process, the reduction in transfer efficiency can produce a ghost image and color mixture due to reverse transfer as well as color difference.
Though toner particles are coated with external additives to improve the charging property, a several percent of a toner is liberated mother particles insufficiently coated with external additives as described above. In addition, in case of a single-component developing method, parts of external additive released from toner surfaces or embedded into resin mother particles due to the repetition of circulation, thus reducing the amount of external additives on the surfaces of the toner particles. Such liberated mother particles and toner particles deteriorated due to the repetition of circulation may have reduced charging property so as to produce defects in charging, leading to reduction in transfer property from photoreceptor to the intermediate transfer member by triboelectric force and thus generating residual toner particles of the apparatus. The adhesive force of the liberated mother particle or the deteriorated toner particle relative to a member may be so large to undesirably cause reverse transfer that toner particles are transferred from the intermediate transfer member to the photoreceptor when the toner particles arrive in the nip portion again. In a conventional image forming apparatus, residual toner particles after transfer are removed by a cleaner. On the other hand, a cleaner-less image forming apparatus permits of neither generation of residual toner particles nor reverse transfer.
Since liberated mother particles and deteriorated toner particles have reduced charging property and reduced transfer property, residual toner particles may be generated. The reduction in transfer property of toner particles can cause various problems. For example, toner particles of different colors may not be superposed on the other and separately adhere to the photoreceptor and to the intermediate transfer member during superposition of multi-color images on the intermediate transfer member (this phenomenon will be called xe2x80x9ccolor-to-color separationxe2x80x9d). In addition, in the next cycle of superposing multi-color images, such toner particles may be reversely transferred from the intermediate transfer member to the photoreceptor. Such phenomenon is a serious problem particular in the cleaner-less image forming apparatus.
The charging property of liberated mother particles and deteriorated toner particles is reduced and the transfer property thereof is thus reduced, leading to generation of residual toner particles. In case that the toner in the form of multi-story adhering to the intermediate transfer member includes normally charged toner particles and insufficiently charged toner particles, separation is caused between the normally charged toner particles and the insufficiently charged toner particles when the toner is transferred to a sheet of paper (this phenomenon will now be called xe2x80x9cstory-to-story separationxe2x80x9d). Residual toner particles are therefore generated onto the intermediate transfer member so as to create hollow defects on the image formed on a sheet of paper, thus leading to degradation in image quality.
In a primary transfer of transferring the toner from the photoreceptor to the intermediate transfer member, a transfer bias is applied to prevent the generation of residual toner particles. When the photoreceptor and the intermediate transfer member are about to separate from each other after the transfer, electric discharge is easily generated particularly at non-image areas because of the application of the transfer bias. Due to this electric discharge, toner particles transferred onto the intermediate transfer member may be charged into an opposite polarity and reversely transferred to the photoreceptor because of a transfer electric field, resulting in color mixture and/or generation of residual toner particles on the photoreceptor. On the intermediate transfer member, toner particles are partially omitted so as to create defects of transferred colorant, sometimes resulting in serious image defects. Such phenomenon may be caused in the secondary transfer.
In the secondary transfer of transferring the toner from the intermediate transfer member to a receiving medium, a transfer bias is applied to prevent the generation of residual toner particles. When the transfer bias is applied, an electric discharge easily occurs at a transfer portion. Toner particles transferred onto the receiving medium may be charged into an opposite polarity due to ions generated by the electric discharge and returned to the intermediate transfer member (reverse transfer) because of the transfer electric field so that residual transfer particles are generated on the intermediate transfer member and that toner particles are partially omitted to create defects of transferred colorant on the receiving medium, resulting in serious image defects.
It is an object of the present invention to obtain stable transfer property regardless of the repetition of circulation of toner particles.
It is another object of the present invention to prevent the generation of residual toner particles to accomplish the cleaner-less arrangement of an image forming apparatus.
It is still another object of the present invention to prevent the generation of residual toner particles, color-to-color separation of a toner, and reverse transfer.
It is further another object of the present invention to prevent the occurrence of story-to-story separation of a multi-story toner when transferred from an intermediate transfer member to a sheet of paper, thereby preventing generation of residual toner particles and preventing the degradation in image quality.
It is yet another object of the present invention to prevent the creation of defects of transferred colorant created due to electric discharge at a transfer portion, thereby preventing the creation of serious image defects.
The present invention therefore provides an image forming apparatus of cleaner-less system using toner particles comprising resin mother particles of which surfaces are coated with external additives, wherein the adhesive force between a primary transfer member and the toner particle is larger than the adhesive force between an image carrier and the toner particle and wherein the adhesive force between the primary transfer member and the resin mother particle is larger than the adhesive force between the image carrier and the resin mother particle.
The present invention still provides an image forming apparatus of cleaner-less system using toner particles comprising resin mother particles of which surfaces are coated with external additives, wherein the adhesive force between the resin mother particles is larger than the adhesive force between the resin mother particle and the image carrier.
The present invention yet provides an image forming apparatus of cleaner-less system using toner particles comprising resin mother particles of which surfaces are coated with external additives, wherein the adhesive force between the resin mother particle and the image carrier is smaller than the adhesive force between the resin mother particles and than the adhesive force between the resin mother particle and a transfer member.
Further, the present invention provides an image forming apparatus using toner particles comprising resin mother particles of which surfaces are coated with external additives, in which images with toner particles of different colors are superposed at a transfer portion, wherein the adhesive force between the resin mother particles of at least two different colors is larger than the adhesive force between the image carrier and the resin mother particle.
Furthermore, the present invention provides an image forming apparatus using toner particles comprising resin mother particles of which surfaces are coated with external additives, in which images with toner particles of different colors are superposed at a transfer portion, wherein the adhesive force between the resin mother particle and the image carrier is smaller than the adhesive force between the resin mother particles of at least two different colors and than the adhesive force between the resin mother particle and a transfer member.
Still further, the present invention provides an image forming apparatus having an intermediate transfer member and using toner particles comprising resin mother particles of which surfaces are coated with external additives, wherein the adhesive force between the resin mother particles is larger than the adhesive force between the resin mother particle and the intermediate transfer member.
In addition, the present invention provides an image forming apparatus having an intermediate transfer member and using toner particles comprising resin mother particles of which surfaces are coated with external additives, wherein the adhesive force between the toner particle and the intermediate transfer member or a receiving medium is larger than the adhesive force between the toner particles.
Further, the present invention provides image forming apparatus having an intermediate transfer member and using toner particles comprising mother particles of which surfaces are coated with external additives, wherein the adhesive force between the toner particles is larger than the adhesive force between the toner particle and the intermediate transfer member and wherein the adhesive force between the toner particle and a receiving medium is larger than the adhesive force between the toner particles.